Pipelines for carrying gas or liquid under pressure are ordinarily made of steel in order to withstand the internal pressures required to transport fluids over large distances. Despite the extensive measures taken during the manufacture and installation of the pipe forming the pipeline, corrosion of the pipe may eventually occur. Pipelines buried in the ground are subject to deterioration from electrolytic and biochemical corrosion, cyclical soil stress, cathodic disbonding, and galvanic corrosion resulting from damp soil and from making pipe attachments, such as valves, from metals which are dissimilar to the steel or other material from which the pipelines are made. Over time, these corrosion mechanisms can cause pits and crevices to form in the pipe. Further, the pipeline may be subject to mechanical damage, such as gouging and denting from the machinery used to install the pipeline or to expose, inspect and repair the pipeline after installation. These gouges and/or dents weaken the pipe and may quicken the corrosion process.
The corroded and gouged regions of a pipeline are typically detected through the use of a "smart pig" or through cathodic surveys. Dented portions of a pipeline may be detected through the use of other conventional "pigs" which determine the clearance in the pipeline as they travel therethrough. Once a defective region has been identified, the fluid pressure in the pipeline is reduced, the soil surrounding the pipeline in the corroded region is excavated, and a reinforcing member is applied to cover the deteriorated area. Normally, the pipeline is prepared for the application of the reinforcing member by removing any corrosion protection material which may have been applied to the pipeline, cleaning the surface by shot-blasting and applying a primer. In one repair technique, reinforcing members in the form of a plurality of split steel sleeves are welded or bolted to the pipeline in end to end relationship until the entire deteriorated area of the pipeline has been covered. The pressure of the fluid in the pipeline is then returned to normal and the pipeline is again buried.
Repair techniques employing split steel sleeves have suffered from several drawbacks. Firstly, these sleeves are very heavy, requiring cranes and several men to transport them to the pipeline and into the proper installation position. Further, the sleeves are often welded both longitudinally and circumferentially, a time consuming process which requires highly skilled workers. Moreover, longitudinal welds in the pipeline must first be ground down in order for the sleeves to form a gas-tight or liquid-tight seal with the pipeline, while the extreme heat of the circumferential welding process for applying the sleeves can structurally weaken the pipe.
A recently developed technique for reinforcing pipeline involves the application of a high tensile strength material to the pipeline by winding to form a plurality of convolutions around the defective region. As described in published Canadian patent application No. 2,028,524, the disclosure of which is incorporated by reference herein, the surface of a deteriorated portion of the pipeline is prepared in a conventional manner. A filler material is then applied to fill in any dents, gouges and corrosion pitting to provide the pipeline with a smooth outer surface. As instructed in the aforementioned patent application and as practiced in the field, the filler material is permitted to cure to a rigid state before the reinforcement process continues. When the filler material has cured, an adhesive is applied over the filler material and over the entire circumference of the pipeline in the region of the defect. A coiled band of a high tensile strength composite is wound around the pipeline with a layer of adhesive applied between adjacent convolutions. The pipeline can be brought to normal operating pressures once the adhesive has cured to a sufficient strength.
The installation of the high tensile strength composite in accordance with the foregoing technique does not require any special machinery or equipment and can therefore be performed relatively quickly by unskilled workers having only a moderate degree of training. However, this reinforcement method has on occasion produced inconsistent results in returning the corroded pipeline to its initial burst strength. That is, although this reinforcement method may at times return the pipeline to its initial burst strength, in at least one instance, the use of the same materials and the same steps for installing the high tensile strength composite has resulted in a test pipe which was not adequately reinforced and which therefore burst prior to reaching the nominal burst pressure of the pipe. Moreover, there is no reliable way of determining whether an adequate repair has been made. In view of these unreliable results, pipeline operators have hesitated to adopt this repair technique.
Accordingly, while different techniques for repairing and reinforcing defective regions in a pipeline have been developed and used in the prior art, there still exists a need for improvements in pipeline repair and reinforcing methods which are easily performed by unskilled workers and, in particular, which restore the burst strength of the pipeline to at least its initial design value on a consistent and reliable basis.